Variable displacement vane pumps are well known and can include a displacement adjusting element, in the form of a pump control ring that can be pivoted or moved to alter the rotor eccentricity of the pump and hence alter the volumetric displacement of the pump. If the pump is supplying a system with a substantially constant orifice size, such as an automobile engine lubrication system, changing the displacement volume of the pump is equivalent to changing the pressure produced by the pump.
Having the ability to alter the volumetric displacement of the pump to maintain an equilibrium pressure is important in environments such as automotive lubrication pumps, wherein the pump will be operated over a range of operating speeds. In such environments, to maintain an equilibrium pressure it is known to employ a feedback supply of the working fluid (e.g. lubricating oil) from the output of the pump to a control chamber where the pressure of the working fluid is used to generate a force, either directly or via a moveable piston, to move the control ring, typically against a biasing force from a return spring, to alter the displacement of the pump.
When the pressure at the output of the pump increases, such as when the operating speed of the pump increases, the increased pressure in the control chamber is applied to the control ring, either directly or via a piston, to overcome the bias of the return spring and to move the control ring to reduce the displacement of the pump, thus reducing the output volume and hence the pressure at the output of the pump.
Conversely, as the pressure at the output of the pump drops, such as when the operating speed of the pump decreases, the decreased pressure supplied to the control chamber allows the bias of the return spring to move the control ring to increase the displacement of the pump, raising the output volume and hence pressure of the pump. In this manner, an equilibrium pressure is obtained at the output of the pump.
The equilibrium pressure is determined by the area of the control ring, or piston, against which the working fluid in the control chamber acts, the pressure of the working fluid supplied to the chamber and the bias force generated by the return spring.
Conventionally, the equilibrium pressure is selected to be a pressure which is acceptable for the expected operating range of the engine and is thus somewhat of a compromise as, for example, the engine may be able to operate acceptably at lower operating speeds with a lower working fluid pressure than is required at higher engine operating speeds. In order to prevent undue wear or other damage to the engine, the engine designers will select an equilibrium pressure for the pump which meets the worst case (high operating speed) conditions. Thus, at lower speeds, the pump will be operating at a higher capacity, supplying a greater pressure of working fluid than required for those speeds, wasting energy pumping the surplus, unnecessary, working fluid.
It is desired to have variable displacement vane pumps which can provide at least two selectable equilibrium pressures in a reasonably compact pump housing.